This invention relates to governors and governor systems for controlling hydraulic transmissions employed in constant speed drives.
Representative prior art includes the commonly assigned U.S. Pat. No. 3,370,600 issued Feb. 27, 1968, to Peterson.
Governors of the type disclosed in the above identified Peterson patent have been employed commercially with a great deal of success in connection with controlling hydraulic transmissions to achieve a constant speed drive condition. In the use of such governors, it is desired that two general conditions be met. The first is during startup, at which time it is desired that the governor direct the transmission to change from a full underdrive condition to a full overdrive position at as low an input speed as is possible, normally by directing hydraulic fluid under pressure to a control cylinder for the wobbler in the typical hydraulic transmission.
The second condition is the shutdown of the system, that is, directing the transmission to shift from any condition to a full underdrive condition when the governor senses a speed equal to or less than some predetermined underspeed value.
As will be appreciated, in both cases an underspeed condition will be present, but opposite results are desired, that is, shifting from an underdrive condition to an overdrive condition in the first instance and, in the second instance, shifting from an overdrive condition to an underdrive condition. In the prior art devices, these shifts typically occur at nearly the same speeds. For instance, as disclosed in the aforementioned Peterson patent, a "failsafe" spring (that designated 101 in Peterson) keeps a governor valve, which controls shifting of the transmission, from moving initially during startup until some predetermined input speed is reached. This predetermined speed is set by the load of the spring and, typically, is generally in the range of 0.2 to about 0.4 pounds of force. Even with such light loads, in the usual system, governor control of the constant speed drive does not occur until the same has achieved about 2000 rpm. At the same time, such loads are sufficiently light as to require fairly drastic deceleration of the governor to overcome the forces generated by the flyweights and viscous draft forces before shutdown will occur.
Thus, while commercially available governors have performed their functions well, there has been a tradeoff in selecting the speed at which the shifting is to occur so that the actual startup speed may be greater than some lower speed at which there would be a minimum shock to the transmission during shifting, this actual startup speed being chosen in order to provide for the shutdown function. At the same time, in order to prevent excessively high accelerative shocks to the system during startup, the shutdown procedure cannot be initiated at as high a velocity as desired.